1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a laser printer, which adopts electrophotography or an electrostatic recording method, and more particularly, to an improvement in an image forming apparatus which temporarily transfers onto an intermediate transfer belt visible images formed on image carriers from electrostatic color materials (such as toner) and collectively transfer the visible images onto a recording material.
2. Description of the Background Art
In an existing and known image forming apparatus of intermediate transfer type, color toner images formed on photosensitive materials are sequentially and primarily transferred to an intermediate transfer belt, and multicolor toner images thus formed on the intermediate transfer belt are collectively transferred onto a sheet by means of a secondary transfer device (e.g., a collective transfer device) (see Unexamined Japanese Patent Application Laid-open No. Hei-2-213879).
The image forming apparatus of this type usually adopts the following system. For example, as shown in FIG. 10, the collective transfer device comprises an intermediate transfer belt 202 and a transfer roller 203 remaining in pressed contact with the outer surface of the intermediate transfer belt 202. For example, a back-up roller 204 which also serves to extend the intermediate transfer belt is provided behind the intermediate transfer belt 202 opposing the intermediate transfer roller 203. A transfer electric field is formed between the transfer roller 203 and the back-up roller 204, thus collectively transferring to a sheet the multicolor toner images formed on the intermediate transfer belt 202. In FIG. 10, reference numeral 201 designates a photosensitive material, and reference number 205 designates a power feed roller which feeds power to, e.g., the back-up roller 204.
The sheet on which the color toner images are collectively transferred is removed (peeled) from the intermediate transfer belt 202 by means of a sheet guide 206 positioned immediately behind the collective transfer device. After having been guided to a sheet transfer device (not shown), such as a transfer belt, provided in the position downstream from the collective transfer device, the sheet is transferred to a fixing device 207.
In the image forming apparatus of intermediate transfer type mentioned previously, because of restrictions on the layout of the intermediate transfer belt 202, the transfer belt 202 is configured so as to make a comparatively large angle .theta.B relative to the direction in which is discharged the sheet that has passed through a nipping area "n" between the transfer roller 203 and the back-up roller 204 in the position downstream from the transfer nipping area "n."
In this type of layout, the intermediate transfer belt 202 forms a contact angle of about 90.degree. relative to the back-up roller 204, which is a power feed member, in the position downstream from the transfer nipping area "n." A comparatively strong electric field is inevitably formed in the vicinity of the exit of the transfer nipping area "n."
The sheet that has passed through the transfer, nipping area "n" inevitably passes through the strong electric field. If an unfixed toner image formed on the sheet is subjected to the influence of the strong electric field, an image formed on the sheet becomes jumbled, thus resulting in picture imperfections (defects) such as retransfer of a toner image to the intermediate transfer belt from the sheet.
In terms of prevention of such image imperfections (image quality defects), it is thinkable that the intermediate transfer belt 202 will be configured so as to make a comparatively small angle relative to the direction in which is discharged the sheet that has passed through the transfer nipping area "n" in the position downstream from the transfer nipping area "n" in order to weaken the electric field produced in the vicinity of the exit of the transfer nipping area "n."
However, in this type of image forming apparatus, it is difficult to ensure a distance between the sheet, which has passed through the transfer nipping area "n," and the intermediate transfer belt 202, thus rendering technical problems apt to arise at low humidity, such as a failure to remove a sheet from the intermediate transfer belt or jumbling of an image resulting from a removal failure.
In fact, if an attempt is made to save space in so-called a tandem image forming apparatus comprising a plurality of photosensitive materials positioned on an intermediate transfer belt in order to ensure productivity, the angle between the direction in which is discharged the sheet that has passed through the transfer nipping area and the intermediate transfer belt situated in the position downstream from the transfer nipping area is set to a small angel, thus rendering technical problems, such as those mentioned previously, likely to occur.